Separate the main package into several internal ones

• Now the compiler's main code is separated into internal/ packages
• Code generation codegen is not depend on ast any more, but on semantic analyzer (sema) and intermediate representation (ir) objects.
• Move a lot of logic from code generator to semantic analyzer
• Adopted DWARF2 debug info https://sourceware.org/binutils/docs/as.html#Loc
• Adopted token.Pos and fileset position mechanism
• Removed pre compiler. It is now generated automatically in test.